Larger barnacle geese (Branta leucopsis) are more efficient feeders: a possible mechanism for observed body size–fitness relationships

Abstract

Geese have to satisfy the high energy demands of flight with a low-energy and bulky food. When feeding on food items that are concentrated and widespread, as is the case in geese, an individual's instantaneous intake rate is largely determined by its bite size. Inter-individual variation in bite size was measured in barnacle geese Branta leucopsis, with the hypotheses that bite size would scale with (sward height)^a, where 0<a<1, and (bill length)^b, where b=3.0, and that there would be a positive interaction between these explanatory variables. Using a generalized linear mixed modelling analysis, bite size was found not to vary over the measured sward heights, whilst bite size was found to scale with bill length to the power b=14.24 (SE=2.05). There was no significantly detectable interaction between these terms, indicating that barnacle geese with longer bills had larger bite sizes over the full range of sward heights studied. Bill length scaled with body mass to the power 0.21 (SE=0.01). Combining this with the scaling of bite size to bill length, we conclude that bite size scales with body mass to the power 2.99. Our results suggest that larger barnacle geese have a disproportionately larger bite size than smaller geese, which may explain the fitness advantages of larger geese observed in other studies. However, smaller geese may resist this selection pressure by selectively consuming more nutritious plant parts or altering their bite rates.